Retroperitoneal and Extraperitoneal Masses: A Localization-First Imaging Approach

A practical radiology review of Retroperitoneal and Extraperitoneal Masses: A Localization-First Imaging Approach, focused on imaging findings, differential diagnosis, reporting points, and high-yield teaching pearls.

Start with localization

Before naming a mass, determine whether it arises from a retroperitoneal organ or from a primary extraperitoneal compartment. A localization-first approach prevents a long, unstructured differential diagnosis.

Signs that suggest organ of origin

• Beak or claw sign: the organ parenchyma tapers around the mass.
• Embedded-organ sign: a mass that arises from an organ may appear embedded within it; an adjacent primary retroperitoneal mass more often compresses and deforms the organ.
• Phantom-organ sign: a very large organ-based mass may obscure the organ of origin.
• Feeding-vessel or vascular-pedicle sign: vessels entering a mass can identify its source.
• Displacement pattern: the direction in which vessels, bowel, kidneys, psoas muscles, and pelvic organs are displaced helps localize the compartment.